Data table merge/load works only when one of the Datatables have a primary key.
Merge can work only when you have specified the primary key. Having an "RowId" column in both datatables isn't enough. You need to set the RowId column as the primary key in at least one of the datatables. Once you've done so, or if this is already in place from a strongly typed dataset/datatable, the call to Merge will work as you want it to.
Let's assume your datatables are called dt1 and dt2 with a primary key of RowId. The following code will do what you want:
Code Snippet
// set primary key prior to merge
dt1.PrimaryKey = new DataColumn[] { dt1.Columns["RowId"] };
dt1.Merge(dt2);
In More detail ...
The Merge method is used to merge two
DataTable objects that have largely similar schemas. A merge is typically used on a client application to incorporate the latest changes from a data source into an existing
DataTable. This allows the client application to have a refreshed
DataTable with the latest data from the data source.
The merge operation takes into account only the original table, and the table to be merged. Child tables are not affected or included. If a table has one or more child tables, defined as part of a relationship, each child table must be merged individually.
The
Merge method is typically called at the end of a series of procedures that involve validating changes, reconciling errors, updating the data source with the changes, and finally refreshing the existing
DataTable.
When performing a merge, changes made to the existing data before the merge are preserved by default during the merge operation. Developers can modify this behavior by calling one of the other two overloads for this method, and specifying a false value for the
preserveChanges parameter.
In a client application, it is usual to have a single button that the user can click that gathers the changed data and validates it before sending it back to a middle tier component. In this scenario, the
GetChanges method is first invoked. That method returns a second
DataTable optimized for validating and merging. This second
DataTable object contains only the
DataRow objects that were changed, resulting in a subset of the original
DataTable. This subset is generally smaller and thus more efficiently passed back to a middle tier component. The middle tier component then updates the original data source with the changes through stored procedures. The middle tier can then send back either a new
DataTable that includes original data and the latest data from the data source (by running the original query again), or it can send back the subset with any changes that have been made to it from the data source. (For example, if the data source automatically creates unique primary key values, these values can be propagated back to the client application.) In either case, the returned
DataTable can be merged back into the client application's original
DataTable with the
Merge() method.
When merging a new source
DataTable into the target, any source rows with a
DataRowState value of
Unchanged,
Modified, or
Deleted, is matched to target rows with the same primary key values. Source rows with a
DataRowState value of
Added are matched to new target rows with the same primary key values as the new source rows.
Please check the following link for further information...
http://msdn.microsoft.com/en-us/library/fk68ew7b%28VS.90%29.aspx#CommunityContent
